1. Field of the Invention
A non-limiting aspect of the present disclosure relates to a tolerance detection method and a tolerance detection device for a shape measuring apparatus. More specifically, the present disclosure relates to a tolerance detection method and a tolerance detection device for a shape measuring apparatus that is suitable to be used for a circularity measuring apparatus and is capable of effortlessly detecting, with one calculation operation, a tolerance of a shape of a measured object having a complicated shape that makes it difficult to perform a continued measurement, and had been calculated only by divisional measurements.
2. Description of Related Art
In order to measure an object having a rotating columnar or cylindrical shape, a shape measuring apparatus, such as a circularity measuring apparatus, is known that collects from such a measured object, various data related to circularity including circularity, concentricity, and coaxiality. When using such a circularity measuring apparatus, a measured object is placed on a turn table. The surface shape of the measured object is detected by a detecting head and the like while the turn table is rotated. Accordingly, the surface shape data of the measured object is accumulated and a value such as circularity is measured and calculated (Related Art 1).
In particular, a stylus having a spherical gauge head at its extremity is biased in a radius direction of the turn table (referred to as R axis direction) and is contacted on a surface of the measured object. A displacement amount of the stylus is detected by a linear encoder while the rotation angle of the turn table is detected by the rotary encoder. By pairing both of the detected values as detection data, the detection data is collected while the measured object is rotated with one revolution, which makes it possible to measure the shape of the entire periphery. Further, the collected detection data is used to perform a minimum square method, minimum domain method, or the like in order to obtain further strict average circular data, which will be used to calculate a circularity value and the like.
As shown in FIG. 1, however, when measuring an object such as a measured object 24, whose shape makes it difficult to perform a continued measurement, due to its protrusions 24a or cut-out portions for key holes or serrations, a circularity measurement apparatus that does not have a profiling measurement function would need to divide the measurement into four partial circumferences as shown in measurements (1)-(4) of FIG. 1, in order to minimize physical damages on the detector. Therefore, a geometrical tolerance can only be obtained to each of the partially divided circumferences. In other words, while the detector that detects displacement is provided with the stylus at its extremity, the stylus only moves in the R axis direction. Therefore, only the displacement in the R axis direction is detected. Accordingly, when there is a projection, the stylus may not be able to move over the projection and break the detector. In addition, when there is a groove, the stylus may become trapped therein and may not be able to emerge therefrom, which may also cause the breakage of the detector.
Accordingly, in order to obtain the geometric tolerance of the entire shape, one must first obtain individual geometric tolerance values of each of the partially divided circumferences, and estimate the geometric tolerance of the entire shape using the maximum and minimum values of the individual partial data.
Related Art 2 describes another method of calculating circularity of a cross section shape having cut-out portions, by removing concave bottom portion data and convex apex portion data that are not subject to the measurement, and calculating the circularity based on the remaining measurement data.
[Related Art 1] Japanese Patent No. 2701141
[Related Art 2] Japanese Patent Laid-Open Application No. H06-11336
However, the above-described method is for a measured object that allows continued measurement of its entire shape. The method cannot be applied to a measured object having a complicated shape that makes it difficult to perform a continued measurement, thereby requiring divisional measurements.